The operating principle of a wire-cut electric discharge machine is that a predetermined gap is maintained between a wire electrode and a workpiece, while a voltage is applied therebetween to cause the spark discharge across the gap, whereby the workpiece is cut by the discharge energy. Accordingly, when the workpiece is moved relative to the wire on the basis of machining command data, it can be machined into a desired shape. In such a wire-cut electric discharge machine, when the wire electrode 1 advances in a groove 3 in the workpiece 2 in a predetermined direction while cutting the workpiece 2 by virtue of the electric discharge, as shown in an operation explaining diagram of FIG. 1, a pressure attributed to the electric discharge develops between the wire electrode 1 and the workpiece 2 as illustrated in a sectional view of FIG. 2, with the result that the wire electrode 1 is pushed back in the direction of an arrow, namely, in the direction opposite to the advancing direction. Therefore, the wire electrode 1 lies behind the position of wire guides 4, 4. That is, the wire electrode 1 flexes. When performing the electric discharge machining of a straight groove, the flexure does not affect the machining accuracy and is not a serious problem.
In a machining operation for forming a corner part, however, the flexure becomes an important problem. In order to form a groove 3 which consists of a first straight groove L.sub.1 and a second straight groove L.sub.2 orthogonal to the former, as shown in a front view of the machined groove in FIG. 3, a corner part CN needs to be formed at the intersection point of the first and second straight grooves L.sub.1 and L.sub.2 by machining. In forming the corner part CN, after the first straight groove L.sub.1 has been formed by the unidirectional relative movement between the workpiece 2 and the wire electrode 1, this relative movement is changed into the orthogonal direction by a machining command. On account of the foregoing flexure of the wire electrode 1 ascribable to the electric discharge, however, the wire electrode 1 of the discharging part is dragged inwardly of the corner part CN, and unlike the commanded shape (indicated by a solid line), the machining path of the groove 3 deviates on the inner side considerably as indicated by dotted lines, so that the machined shape becomes blunt.
Likewise, when the corner part CN' between the first straight groove L.sub.1 and the second straight groove L.sub.2 is to be machined in the shape of a circular arc as shown in a front view of the machined groove in FIG. 4, the machining path of the corner part CN' becomes a blunt machined shape, indicated by the dotted lines, in contrast to the commanded shape of a solid line, because of the flexure of the wire electrode 1 ascribable to the electric discharge. This necessitates such a countermeasure in which the flexure value of the wire electrode is measured in advance, and in case of machining the corner, the workpiece is moved to a somewhat greater extent in correspondence with the flexure value.